SIMPLE PROCEDURE FOR SIMULTANEOUS RECOVERY OF DISSOLVED INORGANIC ANDORGANIC NITROGEN IN N-15-TRACER EXPERIMENTS AND IMPROVING THE ISOTOPIC MASS-BALANCE
G. Slawyk et P. Raimbault, SIMPLE PROCEDURE FOR SIMULTANEOUS RECOVERY OF DISSOLVED INORGANIC ANDORGANIC NITROGEN IN N-15-TRACER EXPERIMENTS AND IMPROVING THE ISOTOPIC MASS-BALANCE, Marine ecology. Progress series, 124(1-3), 1995, pp. 289-299
We developed a simple and reliable method which allows simultaneous is
otope-ratio analysis of inorganic (DIN) and organic (DON) forms of nit
rogen extracted from seawater. All forms of nitrogen under analysis ar
e converted to ammonium, by diffusion with magnesium oxide, prior to c
ollection on glass-fiber filters appropriate for mass spectrometric as
say of N-15. Oxidized DIN forms (nitrate, nitrite) are reduced to ammo
nium in the presence of Devarda alloy. Conversion of DON to ammonium i
s performed by wet oxidation using potassium persulfate and subsequent
reduction of the nitrate formed. Recovery tests, both for total nitro
gen and N-15 content, showed that this procedure is suitable for appli
cation in (DIN)-N-15-isotope dilution experiments and DON-release stud
ies. Recovery of total nitrogen from DIN and DON was nearly complete (
94 to 97%). The variability in the experimental determination of N-15
abundance was < 2% and < 4% for DIN and DON, respectively. We used the
method to balance the N-15 budget in nitrate and ammonium uptake expe
riments conducted in an oligotrophic area (tropical North Atlantic) by
including, in addition to the substrate (DIN) and biomass (PON) pool,
the DON pool. However, the use of glass-fiber filters (GF/F) for the
collection of particulate matter produced a significant artifact, i.e.
a large amount of small particles (< 0.7 mu m, PON<GF/F; prochlorophy
tes and/or bacteria) passed through these filters and were recovered t
ogether with the DON in a combined pool. While inclusion of this combi
ned pool led virtually to a complete accounting for the N-15 label (99
%) in all samples for nitrate uptake and in those for ammonium uptake
incubated for < 8 h, no mass balance was achieved during ammonium upta
ke lasting 10 to 24 h. We suggest that the N-15 that was still missing
(13%) resulted mainly from bottle containment effects such as ammoniu
m-ion adsorption and/or PON adherence onto incubation bottle walls. Tr
ansfer of N-15 label to the combined pool (nitrate experiment) and to
the DON and PON<GF/F pools (ammonium experiment) represented up to 41,
38 and 20% of the total N-15 taken up as DIN, respectively, and depen
ded strongly upon the length of incubation. Failure to take these path
ways of the missing N-15 into account during traditional N-15 uptake e
xperiments involves risk of substantially underestimating new and rege
nerated production, at least in oligotrophic areas. The latter fact ha
s considerable significance in the design of future N-15 tracer method
ologies.